Fixed get_absolute_url() for comments. Previously, it relied on "view on site"

for admin being reversible or required people to set up their own version. Now
we ship a default pattern in the comments URLs in a way that is unlikely to
clash with anything else.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8614 bcc190cf-cafb-0310-a4f2-bffc1f526a37